doc: Added a minimalistic custom CSS for manpages.
doc/Makefile: Removed the dependency on docbook-xsl.css.

Also, get rid of the ugly hack to "create" it.
doc/sysdb-man.css: Applied tokkee.org-like style.
doc: Use asciidoc instead of a2x and added asciidoc.conf.

This allows to specify custom macros, which is currently used to turn
references to other manpages into links in the HTML output. In general, it
will make it easier to apply customizations.
configure, doc: Added ADOCFLAGS.

This allows the user to specify additional flags for AsciiDoc.